All British race meetings were cancelled on Thursday because of an outbreak of equine flu.
The decision to cancel races at four courses came after three vaccinated horses tested positive at an active racing yard. Horse racing authorities did not identify the yard.
Horses from the yard raced on Wednesday at courses at Ayr, Scotland, and Ludlow, in England, potentially exposing a “significant number of horses from yards across the country” and in Ireland.
“The full extent of potential exposure is unknown and we are working quickly to understand as much as we can to assist our decision making,” said the British Horseracing Authority.
It said it was contacting yards to prevent the movement of horses to limit the risk of infection.
The cancellation follows the identification of a number of cases across Europe. The disease is the most damaging of horse viruses in the UK and can result in high fevers and coughing. It’s highly contagious via airborne transmission.
The meetings due to be held on Thursday were at English race courses Chelmsford City, Doncaster and Huntingdon and Ffos Las in Wales.